詳解：設 \(f(x) = \sqrt x \)，\(f'(x) = \frac{1}{{2\sqrt x }}\)

微分公式 \(f(x + \Delta x) \buildrel\textstyle.\over= f(x) + f'(x)\Delta x\)

令 \(x = 100\)，\(\Delta x =  - 0.6\)

\(\sqrt {99.4}  \buildrel\textstyle.\over= \sqrt {100}  + \frac{1}{{2\sqrt {100} }}( - 0.6) = 10 - 0.03 = 9.97\)